1. Field of the Invention
The present invention relates to a process for producing a rubber compound, and more particularly to a process for producing a rubber compound by radical polymerization of acrylic monomer in the presence of a fluorine-containing elastomer.
2. Related Prior Art
Acrylic copolymer elastomer (acrylic rubber) has good oil resistance and heat resistance, and also is at a relatively low cost and thus has been much used as vulcanization-molding materials such as various automobile sealing materials, etc. Higher temperature of using circumstances due to recent higher efficiency of automobile engines is requiring materials having a much better heat resistance. On the other hand, fluorine-containing elastomer (fluorine rubber) is used particularly at heat resistance-requiring positions, among the automobile engine sealing materials, because of its distinguished heat resistance, but its drawback is a very high cost.
In these situations many attempts to blend the acrylic rubber with fluorine rubber have been made to satisfy both of the heat resistance and cost, but the conventional art of cross-linking these two rubbers by a cocross-linking agent capable of cocross-linking these two rubbers hardly satisfied even the intermediate heat resistance between the heat resistance of the acrylic rubber and that of the fluorine rubber. For the required improvement, it has been regarded as important to make better the compatibility of these two rubbers.
JP-A-4-363352 discloses a process for improving the interdispersibility between a fluorine-containing polymer and an acrylic polymer by dissolving or swelling an amorphous, fluorine-containing polymer into or in an acrylic monomer, followed by polymerization reaction. Polymers obtained from the acrylic monomer are very liable to have a lower molecular weight, resulting in poor processability, and also their content in the rubber compound is low, so that the aim to lower the cost cannot be attained.
JP-A-1-299859 discloses the art of preventing boundary surface pealing by cross-linking substantially only acrylic rubber in blending of fluorine rubber with the acrylic rubber, thereby making the grain size of dispersed acrylic rubber and increasing the interfusion of molecules through the boundary surface between the two rubber, but the process is not suitable for mass production and it is hard to obtain a stable blending simply.
To solve these problems, JP-A-5-287156 proposes a process for blending an internally cross-linked acrylic rubber with a fluorine rubber, but when the internally cross-linked acrylic rubber is used in a higher blending ratio, no satisfactory plasticity is obtained and vulcanization molding is hard to conduct. That is, there is a limit to its blending ratio and the aim to lower the cost cannot be attained.
Known processes for making compatible polymers from non-compatible polymers include a process for forming an IPN (interfusion polymer network) of rubber components by dynamic cross-linking, thereby improving the impact resistance of crystalline resin, a process for seed polymerization (core-shell polymerization) to obtain a paint capable of producing good film characteristics, (JP-A-3-7784), etc., but in case of blend rubber having no thermoplasticity, IPN elastomer has a low plasticity and thus has a molding problem, and also there have no actual cases of applying the seed polymerization process to a non-compatible rubber system of fluorine rubber and acrylic rubber.
JP-A-63-312836 discloses addition of 100 to 500 parts by weight of .alpha., .beta.-ethylenic unsaturated carboxylic acid ester per 100 parts by weight of the fluorocarbon polymer in a primary particle dispersion of fluorocarbon polymer obtained by using a nonionic surfactant and an anionic surfactant, followed by rodical polymerization reaction, where the product is a fluorocarbon polymer compound having a hydrocarbon polymer outer layer and is no such compound that the individual polymers themselves are entangled with one another. The product compound is used as a coating composition and has a good dispersibility in water, an organic solvent, a synthetic resin solution or a synthetic resin dispersion.